There exist apparatus, such as the novel portable and light weight tool disclosed and claimed in commonly assigned Ericsson, U.S. Pat. No. 3,586,572, and Vilcins et al., U.S. Pat. No. 3,669,799, for friction-fusion bonding overlapping portions of a thermoplastic strapping ligature in extremely short period of time. In operation, such apparatus tensions the strap and then frictionally fuses the tensioned overlapping portions of the strap.
Since satisfactory friction-fusion joints are produced quickly, accurate time control of the operation of such apparatus is important. If the fusion operation is terminated too quickly, insufficient frictional heat is generated at the interface of the overlapping portions of the strapping ligature and a joint of inadequate strength is produced. If the fusion operation is prolonged for too long a period of time, excessive heat is generated which again results in the creation of a joint having inadequate strength.
In order to reduce the requirement for a high degree of operator skill, an automatic control circuit is desirable in order that the tool is operated for the appropriate length of time. The necessity for controlling the friction-fusion time is applicable to various types of apparatus for that purpose.
In Frey, U.S. Pat. No. 3,679,519, there is disclosed an electro-mechanical control system for controlling the duration of the operating cycle of a tool for friction-fusing overlapping portions of a thermoplastic strapping ligature. While the control system disclosed in the Frey patent is satisfactory, it can be appreciated that the use of a electro-mechanical control circuits does create problems of reliability and maintenance.
It is therefore highly desirable to utilize a control circuit capable of controlling the overall duration of the operating cycle of a friction-fusion apparatus, capable of adjusting the duration of the operating cycle as desired and which at the same time exhibits a high degree of reliability and minimal maintenance while at the same time utilizing a minimum of space in the tool.